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Only sure way to tell is take the diff cover off and look at the ring gear, rotate it around, you will see 2 numbers, for example 40/13 which is a 3.08. If they are original to the truck and you have the sticker on the inside of the glove box door it will tell you or give you a RPO code like a HE3 for example. Report your findings.http://www.rpocodes.com/GM-RPO-codes/gmrpocodesGthroughM.html should help you know what you have or originally came in the truck